This invention relates to processes and products derived from fly ash. Fly ash is the by-product of burning pulverized coal, and its chemical content, and the size distribution of its particles, vary widely in accordance with the source of the coal, the fineness to which it is ground, and the furnace within which it is burned. The chemical compositions of fly ash can vary widely, as follows:
______________________________________ Component Per cent by Weight ______________________________________ SiO.sub.2 35-55 Al.sub.2 O.sub.3 15-35 FeO Fe.sub.2 O.sub.3 3-25 Cao 3-8 MgO 0.5-3 TiO.sub.2 1-3 Na.sub.2 O K.sub.2 O 1-6 SO.sub.3 1-3 C 0.5-10 H.sub.2 O 0.5-0.7 ______________________________________
These compositional ranges apply to most fly ashes derived from coal, and the composition of the fly ash from a given plant is usually fairly constant with a given source or grade of coal.
Most previous efforts to extract useful components from fly ash have dealt with it in bulk. It has been subjected to various routine ore dressing techniques. With additives it has been reheated and reacted, compressed and fired, pelletized and sintered, fluxed and melted. Various chemical extractions have been tried.
In U.S. Pat. of Pennachetti et al, No. 3,533,819 issued Oct. 13, 1970, certain processes were applied to fly ash in an attempt to provide useful fractions therefrom. The fly ash was handled in its dry state and was air classified to separate the finer fraction from the coarser fraction, and the finer fraction was then used as a pozzolan to replace Portland cement. Magnetic separation was applied to remove a magnetic fraction and to form an iron concentrate product and the coarse fraction was screened to obtain a coarse carbon product. However, in the Pennachetti et al patent, a major portion, if not all, of the original carbon content by percentage was allowed to remain in the pozzolan material, and nothing further was done to the pozzolan material to improve or enhance it.
Magnetic separation of whole fly ash is also known from U.S. Pat. No. 2,987,408 of Minnick, issued June 6, 1961, to provide a non-magnetic fly ash fraction for use as a pozzolan.
The dry ball milling of whole fly ash and lime mixtures to provide an improved pozzolan which is then diluted with additional fly ash prior to use is disclosed in U.S. Pat. of Webster, No. 3,852,084, issued Dec. 3, 1974. In Webster, it is noted that the glassy spheres of the fly ash may be fractured in the ball milling. The fly ash is milled with a relatively large amount of lime, that is from 4 to 90 percent by weight of the premix and from 1 to 40 percent by weight of the final diluted mixture. There is no suggestion in Webster of substantially eliminating the carbon or the iron content fly ash prior to milling, nor is wet processing suggested, nor is any mixture suggested in which the lime component is about one percent or less of the fly ash by weight.